1. Field of the Invention
The present invention relates to a motor control circuit and, more particularly, to a motor control circuit for supplying a controlling driving voltage to a motor.
2. Description of the Related Art
Generally speaking, an H-bridge circuit constructed by four switch transistors may supply a driving voltage to a motor, such as DC motor, stepping motor, voice coil motor, and the like, for controlling a rotating direction, a rate of rotation, and other operational characteristics.
FIG. 1 is a circuit diagram showing a conventional H-bridge circuit 10 for driving a motor M. Referring to FIG. 1, the motor M is coupled between a supply voltage source Vm and a ground potential through the H-bridge circuit 10. Although the motor M is in practice a complex system consisting of a variety of mechanical and electrical components, the driving voltage is essentially applied to a winding of the motor M for generating a magnetic field. Consequently, the term “motor” in this specification primarily refers to the winding of the motor M, which may be reduced to an inductive load for a simpler consideration. The drawing also emphasizes this consideration by illustrating the representative winding of the motor M.
The H-bridge circuit 10 includes four N-channel MOSFET transistors (NMOS) Q1 to Q4. The NMOS transistor Q1 has a drain electrode coupled to the supply voltage source Vm and a source electrode coupled to a terminal A of the motor M. The NMOS transistor Q2 has a drain electrode coupled to the supply voltage source Vm and a source electrode coupled to a terminal B of the motor M. The NMOS transistor Q3 has a drain electrode coupled to the terminal A of the motor A and a source electrode coupled to a ground potential. The NMOS transistor Q4 has a drain electrode coupled to the terminal B of the motor M and a source electrode coupled to the ground potential.
Since the NMOS transistors Q1 to Q4 have parasitic diodes D1 to D4, respectively, the H-bridge circuit 10 needs not to be additionally provided with flywheel diodes. If the four switch transistors of the H-bridge circuit 10 are implemented by bipolar junction transistors, however; the diodes D1 to D4 shown in FIG. 1 should be additionally provided.
The gate electrodes of the NMOS transistors Q1 to Q4 are controlled by control signals G1 to G4, respectively. When the control signals G1 and G4 are at a logic high level and the control signals G2 and G3 are at a logic low level, the NMOS transistors Q1 and Q4 are turned on and the NMOS transistors Q2 and Q3 are turned off such that the terminal A is coupled to the supply voltage source Vm through the conductive NMOS transistor Q1 and the terminal B is coupled to the ground potential through the conductive NMOS transistor Q4. As a result, the supply voltage source Vm applies a driving voltage to the motor M, causing a driving current I1 to flow through the motor M in a direction from the terminal A toward the terminal B. When the control signals G1 and G4 are at the logic low level and the control signals G2 and G3 are at the logic high level, the NMOS transistors Q1 and Q4 are turned off and the transistor Q2 and Q3 are turned on such that the terminal B is coupled to the supply voltage source Vm through the conductive NMOS transistor Q2 and the terminal A is coupled to the ground potential through the conductive NMOS transistor Q3. As a result, the supply voltage source Vm applies another driving voltage to the motor M, causing another driving current I2 to flow through the motor M in another direction from the terminal B toward the terminal A.
Among the various applications of using the motor M, the driving voltage applied between the terminals A and B determines practical operational characteristics of the motor M and therefore needs to satisfy several requirements of application. At first, a polarity and an absolute value of the driving voltage should belong to a controllable quantity because the polarity of the driving voltage determines a direction of the magnetic field generated by the winding of the motor M and the absolute value of the driving voltage determines a strength of the magnetic field generated by the motor M. Especially when the motor M needs to be operated in a constant voltage driving condition, the absolute value of the driving voltage must be kept constant.
Conventionally, a pulse width modulation (PWM) technique is usually adopted to control the absolute value of the driving voltage applied to the motor. More specifically, in the situation where the NMOS transistor Q4 is turned on and the NMOS transistors Q2 and Q3 are turned off, the control signal G1 may be implemented by a PWM signal such that the ON time of the NMOS transistor Q1 is determined by the duty cycle of the PWM control signal G1, thereby controlling an average value of the driving voltage. However, the PWM technique inevitably induces excessive disturbances in the supply voltage source Vm and the driving voltage applied to the motor M. For the application which requires extremely precise control of the motor, the conventional PWM technique may cause several disadvantageous effects. Therefore, it is desired to provide a control circuit capable of supplying a low noise driving voltage to the motor M.